Handheld vacuum cleaners are well known and have been manufactured and sold for several years. Typically, a handheld vacuum cleaner comprises a body which houses a motor and a fan unit for generating an airflow. The airflow enters the vacuum cleaner via an inlet. A separator such as a filter, bag or cyclonic separator is located downstream of the inlet to separate dirt and dust from the airflow. An example of this type of vacuum cleaner is shown in GB 1 207 278.
It is common for a handheld vacuum cleaner to be battery powered. In many cases, a rechargeable battery is provided. However, a known disadvantage of battery powered vacuum cleaners is that the voltage supplied by the battery can drop significantly during use. Further, the rate at which the voltage drops increases as the charge remaining in the battery decreases. A reduction in battery voltage may result in a reduction in motor speed, leading to a reduced airflow through the vacuum cleaner and a corresponding reduction in the cleaning performance of the vacuum cleaner. This is inconvenient for a user because the vacuum cleaner will perform a less effective cleaning operation.
JP 2001-353111 discloses a rechargeable battery arrangement for driving a motor of a vacuum cleaner. The arrangement is able to compensate for a reduction in current supplied by the battery as the battery is drained in use. This is achieved by varying the duty of the current as the current from the battery reduces in order to supply a constant current to the motor.